The invention concerns a method and a device for controlling the phase relation between a head drum and tape feed system of a magnetic tape recording device.
In the search operation of an oblique track recording device, read- and recording track angles differ from one another. This has the result that due to the azimuthal decoupling only track segments can be read. In analogue recording devices, operating for example in accordance with the VHS standard, horizontal interference stripes occur. By selecting the speed, the number of visible interference stripes is determined (5-fold=4 interference stripes). In digital recording systems this effect likewise occurs. In the case of video recording devices with considerable segmentation (e.g. 12 recording tracks per frame) and data reduction (e.g. DCT=discrete cosine transformation) the picture correlation of the data of two adjacent tracks is no longer present. Furthermore the data have a greater weighting according to the reduction factor so that the absence of entire data groups can lead to a total dropout in some parts of the picture.
The recorded useful data signal is embedded in the normal way in synchronizing words, error protection data and pointers which indicate where the following data packet, together with the error protection data, belongs in the picture. The smallest readable quantity of information is thus a xe2x80x9csynchronizing blockxe2x80x9d, the data of which is input into a store. In the event of a dropout during the reading of new data, the whole synch-block is lost and for error recovery purposes recourse can be had to the old information.
In search operation of such a digital recording system the undesired effect arises that with specific constellations of head drum servo and tape feed servo during search operation. the same data zones are always read from the tape and updated on the screen, while other data zones are not read from the tape and the corresponding picture areas retain the old content. This is extremely disturbing in practice.
The aim of the invention is to reduce the disturbances which occur during search operation. This aim is fulfilled by the features of the invention described in Claim 1. Advantageous further developments of the invention are described in the sub-claims.
In accordance with the invention, the head drum phase and tape feed phase is controlled via the tape feed in such a manner that all the pointers are input equally often in an interval comprising a plurality of consecutive tracks=frames. In the case of the segmentation of 12 tracks for each frame with in each case 149 pointers (for the known DV-Standard), this means a search criterion for 1788 different items of stored information, from the frequency distribution of which the phases must be adjusted for a plurality of speeds. In an embodiment of the invention, on the recognition of an address information item (pointer) with associated data group, a store location in a measured value store is incremented or decremented, and the content of all the measured value store locations is continuously monitored and in dependence upon the count a change in phase of the tape feed is carried out.
Different solutions are possible in accordance with the invention for monitoring the measured value store locations and these will be described in the following.
Each item of address information (data pointer) is allocated a separate store location. The counter content of all the store locations per time unit should attain the same value, where the time units are determined by the respective operating mode. This is achieved by reading the counter value of each store location in a xe2x80x9cround Robinxe2x80x9d fashion, i.e. successively from pointer number 0 to 1787 and feeding this counter value to a digital/analog converter where the corresponding analog value is generated. The output signal of the DAC will be the analog representation of the accumulated pointer values within a given time frame. In the analog output signal areas with high voltage will occur where the pointer values have been read often. Low voltages will occur where the pointer values have been read seldom or never. This analog signal is fed to the servo control circuit. There, the signal is analyzed with respect to regions having high or low voltages. From the differences in height of the signal and in distances between the high and low voltages regions a correction value is calculated which will be used to generate a corresponding control voltage pulse which changes the phase relation so that preferably those data regions are read which have not been updated or seldom been updated.
Here, groups of pointer values are combined, so that measured value store locations are saved. The dropouts of the data signal read from the tape determine which pointer values are combined to a group. The first item of pointer value recognized after a dropout is stored as value 1 and the last item of pointer value recognized prior to a dropout is stored as value 2 in the measured value store together with the frequency of their occurrence. The change in phase of the tape feed is controlled digitally via digitally servo control in such manner that the occurring gaps are to be purposively filled up.
The dropout criterion is obtained from a PLL and/or from an envelope curve detector and/or a loss of synchronizing signal triggers the dropout criterion. From the calculated difference between the stored values 1 and 2 of consecutive data groups an increment size, by which the tape feed phase is to be changed, is calculated and used to control the phase relationship. The frequency value of occurrence of the data groups can be additionally taken into account for the calculation of the increment size.
This solution presupposes that low resolution pictures are additionally recorded for the different possible search modes in special purpose data areas of the tracks. Then in this selected search modes the phase is controlled in such a manner that the differences between the currently read pointer values and the wanted pointer values of the special purpose data areas are calculated. From this difference value a phase correction value is defined and output.